If you are trading a Pokémon to a friend and you wish to give them a message, you have the ability of attaching a mail to the Pokémon. The mail come in a variety of styles. In the original Gold & Silver games, you could manually write the mail and place anything you wanted on it. However, in later games, it got restricted to to a massive selection set phrases. The mail is all one-use only and if you remove it from a Pokémon, the mail gets sent to your in-game PC.